When most people think about searching for extraterrestrial intelligences, they imagine someone like Ellie Arroway searching the skies for radio transmissions. But what about looking in other ways? Perhaps a highly advanced alien civilisation might build structures large enough for us to see.

Vast structures, constructed on astronomical scales by advanced civilisations, is what the field of astroengineering is all about. This, admittedly, sounds audacious – and for the human race right now, it is. For us, astroengineering is still very much the realm of thought experiments, theoretical calculations, and science fiction. So it may be surprising to know that certain astronomers have made some quite serious attempts to look for astroengineered artifacts around other stars. With telescopes becoming ever more sensitive, and images being taken of exoplanets, the idea is starting to captivate imaginations once more.

In 1960, Freeman Dyson published a paper entitled Search for Artificial Stellar Sources of Infrared Radiation. His suggestion was that any megastructure constructed around a star should show itself by emitting more infrared light than it should. The solution was, simply, to look for any sources of infrared which appeared artificial.

Dyson put forward the ideas that any potentially advanced civilisation may need a tremendous amount of power to sustain itself. A method he proposed was to build a vast array of satellites which would enclose an entire star to harvest its energy – a concept which later came to be known as a Dyson sphere. While the concept wasn’t taken too seriously by Dyson himself, it was a powerful enough notion that it garnered a lot of attention.

Unfortunately, it isn’t as simple as looking for infrared light. Many stars, our own included, are surrounded by a disk of dust, and that dust emits plenty of infrared. To find a Dyson sphere, you need to look for a specific signature of infrared light, emitted at just the right set of wavelengths.

But infrared isn’t the only way to spot Dyson spheres. In 2012 Geoff Marcy, an exoplanet researcher, was given a grant to hunt for evidence of Dyson spheres in data recorded by Kepler. In principle, any large artificial objects in orbit around other stars should be detectable in exactly the same way exoplanets are.

Artificial Worlds

Estimated sizes of the two Death Stars, compared with Earth and the Moon. Credit: Invader Xan /supernovacondensate.net

Astroengineering is, quite obviously, no easy feat; the larger the artefact being engineered, the more difficult it would be to create. For example, consider the infamous Death Star from the Star Wars movies. No official size was given in the original movie, but it’s been estimated to have been a few hundred kilometres in diameter (depending on who you’re asking), meaning that none of the engineering materials currently used here on Earth are strong enough to survive the kind of stresses involved in actually constructing one. In other words, White House petition or not, any attempt we were to make at constructing a Death Star sized object would crumple and break apart long before it was completed.

While the Death Star may be iconic in many of our minds as a vast artificial structure, it’s actually fairly small by astroengineering standards. Even the most grandiose estimates of how large it was are still small even compared to Earth’s moon, meaning something this size would be still be difficult to spot with a telescope. If we were to be able to actually spot an astroengineered structure, it would need to be far grander.

Similar to Dyson spheres are ringworlds, like those depicted in the Halo video games. A ringworld would consist of a giant ring in orbit around a star, constructed comfortably inside the star’s habitable zone. This would give an advanced civilisation a habitat with an area several million times the size of Earth in which to live. While a planet like our own could suffer as it begins to strain under the weight of an increasing population, an advanced society could have a population of trillions living on a ringworld.

Thinking on scales any larger, and things begin to become speculative, and some radical ideas have been put forward. For instance, a matrioshka brain would use a series of concentric Dyson spheres to harvest a star’s entire energy output to power a vast computer. Equally mind-bending is the idea of the Shkadov thruster which would use a gigantic mirror as a sail, enabling an entire star to be accelerated and moved, together with anything which happened to be orbiting it. But let’s not get ahead of ourselves.

One thing is quite certain – For any alien race to even consider an astroengineering project, they’d need to be dramatically more advanced than we are. Of course, as some scientists have pointed out, “There is no obvious reason why our own astronomical epoch favours the emergence of civilizations”. In other words, our galaxy is billions of years old, and there’s no reason to believe that advanced alien societies haven’t already been in existence for a long time before we humans had even evolved.

Could Dyson spheres and other astroengineered megastructures really exist elsewhere in the galaxy? Currently, we have no way of knowing. But the possibility alone is exciting enough to make it worth continuing to look. Perhaps if we find the right star to look at, we might just find something amazing.